For the pyrotechnic production of nitrogen-rich gases, attention has for a long time been paid to pyrotechnic compositions based on sodium nitride, for example those described in U.S. Pat. No. 4,929,290. However, compositions based on sodium nitride have a number of drawbacks. Firstly, when they combust, these composition generate a great deal of solid dust. It is therefore necessary, when this type of composition is used, to equip the gas generator with a sophisticated filtration device which increases the weight and cost of the generator. Furthermore, nitride-based compositions are very sensitive to humidity and have poor long-term stability. Lastly, since sodium nitride can cause primary explosions by combining with metals, for example to form lead nitride, these compositions require that precautions be taken during their manufacture and when disposing of waste.
For all these reasons, the person skilled in the art has sought to avoid sodium nitride and has targeted solid compositions which generate nitrogen and consist of a binder and an oxidant filler, which compositions have very good long-term stability.
The use of this type of composition as propellant for rocket and missile motors is known and has been described in a number of patents, for example U.S. Pat. No. 3,725,516 which describes rocket motor propellants consisting of a binder containing fluorine, an oxidant salt such as ammonium nitrate or potassium perchlorate and a metal in powder form.
However, such propellants cannot be used in the applications to which the present invention relates for at least two reasons. On the one hand, they contain a very high proportion of binder, often between 10 and 35%, and when they combust they therefore produce large amounts of highly toxic gases, and on the other hand they employ metal compounds which generate large amounts of solid residues.
In order to attempt to overcome these difficulties, the person skilled in the art has therefore proposed compositions based on ammonium nitrate and/or potassium perchlorate in combination with derivatives of tetrazole and metal compounds such as boron oxide, vanadium oxide or silicates. Such compositions which are, for example, described in U.S. Pat. No. 5,035,757 do indeed give nitrogen-rich non-toxic gases, but nevertheless produce a great deal of solid residues which make it necessary to have a powerful filtration system present.
Compositions based on cellulose acetate, ammonium nitrate and potassium perchlorate have also been proposed, for example in U.S. Pat. No. 5,462,579. Although such compositions do indeed lead to nitrogen-rich gases, in view of their high proportion of binder, these gases are often toxic because they are too rich in carbon monoxide. Furthermore, to achieve satisfactory combustion rates, the person skilled in the art is in practice forced to add metal oxides or metal powders, as indicated in this patent, and therefore is again confronted with the need to filter the combustion gases.
Compositions which expressly exclude the presence of metal compounds, and essentially comprise a thermoplastic binder containing oxygen, an energetic plasticizer such as polyglycidyl nitride and an oxidant filler composed for at least 85% of its weight by ammonium nitrate, have been proposed in U.S. Pat. No. 5,525,171.
However, in order for the charges formed in this way to have satisfactory mechanical strength, both immediately following manufacture and after prolonged ageing, it is in practice necessary for the composition to contain at least 8% by weight of binder, not to mention the energetic plasticizer and, at least for certain applications, the person skilled in the art is once again confronted with the problem of the gases' toxicity.
Compositions whose oxidant filler is based on ammonium nitrate and which contain less than 6% by weight of an organic binder containing oxygen and between 0.5% and 5% by weight of a light metal selected from the group consisting of boron and aluminium have therefore been proposed in patent application U.S. Ser. No. 08/874,634. These compositions, which are advantageously formed into pellets by compression, have a satisfactory combustion rate and do not produce heavy solid residues when they combust, and the gases which are generated have an extremely low level of carbon monoxide. Nevertheless, the gases which these compositions generate have a level of nitrogen oxides which is still a little too high when they are intended to operate in systems which are open to the outside.